The present invention relates to a phase shifting mask and a method of manufacturing the same, and more specifically, to an alternating phase shifting mask using a flowable oxide film, in particular, and a method of manufacturing the same.
Of the phase shifting masks, the alternating phase shifting mask is known as one of the effective lithographic techniques capable of achieving a high resolution (for example, M. D. Levenson et al. IEEE Trans. in Electron Devices, Vol. ED-29, No. 12 (1982) p. 1828). However, such a process is completely different from the process for ordinary COG (chromium on glass) masks, and is not easy to achieve.
The alternating phase shift mask can be categorized into mainly two types, in terms of the process for forming the shifter. One is an etched substrate shifter type (subtractive shifter type) such as shown in FIGS. 1A and 2, and the other is a film shifter type (additive shifter type) such as shown in FIGS. 3A and 4. Since it is very difficult to etch quartz, which is the material for the substrate, in an optically ideal etched profile, generally, the former is more difficult to manufacture than the latter.
An alternating phase shifting mask is used generally together with a negative-type resist, in order to avoid unnecessary resist patterns with shifter edge portions. In connection with an etched substrate shifter type mask, when it has a line pattern (island-like pattern) such as shown in FIG. 1A, the section other than the lines is covered by a chromium opaque film. In the line pattern sections (open sections of the opaque film), shifters are alternately provided. A shifter is formed by engraving a substrate as can be seen in FIG. 2. When a negative-type resist on a wafer is developed by an exposure, using the obtained mask, such a resist pattern as shown in FIG. 1A can be obtained.
In the case where the same pattern is formed with the film shifter type mask, shifter films are alternately formed on the opening sections of the chromium opaque film, as shown in FIGS. 3A and 4. In this type, since a shifter section and a non-shifter section are not arranged to be directly adjacent to each other, a shifter film may be extruded out on the opaque film, and with such an allowance, the alignment between the film and opening sections becomes very easy.
The SOG (spin-on-glass) coating on mask, for example, is very popular for the manufacture of film sifter type phase shifting films. This is because the thickness of an SOG film can be easily controlled, and the optical characteristics (especially, refractive index) thereof are close to those of a quartz substrate.
However, even an alternating phase shifting mask which employs such an SOG film entails several drawbacks which render the process thereof very difficult. These drawbacks are mostly concerned with the etching step for removing the SOG film in a non-shifter region.
FIGS. 5A to 5D are cross sectional views showing a process of manufacturing an SOG coating-type alternating phase shifting mask, step by step. First, as shown in FIG. 5A, a chromium opaque film pattern is formed on a quartz substrate 11. Then, an SOG film 13 is formed on the entire surface of the substrate. Subsequently, as shown in FIG. 5C, a resist pattern 14 is formed, and using the resist pattern 14 as a mask, the SOG film 13 is etched, thus forming a phase shifting mask as shown in FIG. 5D. For this etching process, wet etching process or reactive ion etching process are available.
In this case, the quartz substrate 11 may be etched to a certain extent as well during the formation of the non-shifter region 15 for both etching process. This unexpectedly etched portion may generate a shift from the desired phase difference of 180 degree between the shifter region 16 and the non-shifter region 15, which results in an uneven profile of an image intensity as shown in FIG. 5E.
When the image intensity is low, a pattern cannot be formed, or if it is formed, the pattern width is reduced, or a residue remains with the result that a sufficient resolution can not be obtained as a phase shifting mask. This insufficient resolution reduces the process margin with respect to the lithography step in the manufacture of a semiconductor device. This causes, for example, an device error such as short-circuiting of a wiring, which lowers the yield of the product.
In order to avoid such a drawback, a mask forming method as shown in FIGS. 6A to 6D is considered. In this method, an etching stopper (for example, Al.sub.2 O.sub.3) 17 is formed on the quartz substrate 11. In these figures, the other members which are similar to those shown in FIGS. 5A to 5D, are designated by the same reference numerals, and the duplicated explanations therefor will be omitted since these methods are very close to each other.
However, the mask blank of this kind is very expensive, and an etching damage 18 in a non-shifter region 15 on a stopper 17 may cause an unevenness in the image intensity as shown in FIG. 16.